Aqueous polymer dispersions for coating wood

ABSTRACT

Aqueous polymer dispersions essentially containing a copolymer of 
     a) from 85 to 98.5% by weight of methacrylates, 
     b) from 0.5 to 4% by weight of arcylic acid and/or methacrylic acid, 
     c) from 0.5 to 4% by weight of acrylamide and/or methacrylamide, 
     d) from 0.5 to 4% by weight of an α,β-ethylenically diunsaturated or polyunsaturated compound and 
     e) from 0 to 3% by weight of a carbonyl-containing α,β-ethylenically unsaturated compound, a process for their preparation, and their use for coating wood.

This application is a continuation of application Ser. No. 141,100, filed on Jan. 5, 1988, now abandoned.

The present invention relates to aqueous polymer dispersions essentially containing a copolymer of

a) from 85 to 98.5% by weight of methacrylates,

b) from 0.5 to 4% by weight of acrylic acid and/or methacrylic acid,

c) from 0.5 to 4% by weight of acrylamide and/or methacrylamide,

d) from 0.5 to 4% by weight of an α,β-ethylenically diunsaturated or polyunsaturated compound and

e) from 0 to 3% by weight of a carbonyl-containing α,β-ethylenically unsaturated compound,

and a process for its preparation and its use for coating wood.

Wood surfaces, such as those of paneling, furniture and doors, are frequently coated with transparent finishes for protection and for aesthetic reasons. To date, nitro finishes, acid-curable alkyd-amino finishes, finishes based on unsaturated polyesters, and polyurethane finishes have been used. The stated traditional finishes are unacceptable for reasons relating to occupational hygiene and environmental protection, because of the solvents which are present therein and which pass into the atmosphere during processing and drying.

German Laid-Open Application DOS 2,535,660 describes a process for the preparation of crosslinkable polymers and copolymers containing methylolated carboxamide groups. European Patent 29,144 discloses the use of N-methylol(meth)acrylamide-containing aqueous polymer dispersions for coating wood surfaces. The methylol groups may react with one another at above 100° C., i.e. the finish may undergo crosslinking and hence an improvement in its resistance to chemical influences. However, the disadvantage is that the temperature required for the reaction of the methylol groups is frequently too high when the substrate is wood, and that formaldehyde is eliminated in this crosslinking reaction.

Furthermore, these coatings give surfaces which do not fully meet practical requirements with regard to scratch resistance, hardness, flexibility, sandability, stackability and resistance to chemical influences, for example drinks and cleaners.

It is an object of the present invention to provide polymer dispersions for the preparation of aqueous wood finishes which do not have the abovementioned disadvantages.

We have found that this object is achieved by aqueous polymer dispersions essentially containing a copolymer of

a) from 85 to 98.5% by weight of methacrylates,

b) from 0.5 to 4% by weight of acrylic acid and/or methacrylic acid,

c) from 0.5 to 4% by weight of acrylamide and/or methacrylamide,

d) from 0.5 to 4% by weight of an α,β-ethylenically or polyunsaturated compound and

e) from 0 to 3% by weight of a carbonyl-containing α,β-ethylenically unsaturated compound.

Particularly suitable components a) are methacrylates of C₁ -C₁₂ -monoalcohols, preferably C₁ -C₈ -monoalcohols, such as methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, tert-butanol, cyclohexanol, 2-ethylhexanol and lauryl alcohol. n-Butyl methacrylate or a mixture of n-butyl methacrylate with up to 25% by weight of methyl methacrylate is particularly preferred. The methacrylates are used in amounts of from 85 to 98.5, preferably from 90 to 97, % by weight.

Components c) and d) are each used in amounts of from 0.5 to 4, preferably from 0.5 to 2.5, % by weight.

α,β-ethylenically diunsaturated or polyunsaturated compounds are used as component d), in amounts of from 0.5 to 4%, preferably from 0.5 to 2.5, % by weight. Examples of suitable compounds are divinylaromatics, such as divinylbenzene, diacrylates and dimethacrylates of

C₂ -C₂₄ -diols, such as butanediol diacrylate, butanediol dimethacrylate and hexanediol diacrylate, divinylureas, such as divinylethyleneurea, and di- and triallyl compounds, such as diallyl phthalate and triallyl isocyanurate. Divinylbenzene, butanediol diacrylate and butanediol dimethacrylate are preferably used.

Carbonyl-containing α,β-ethylenically unsaturated compounds e) may also be present. Where they are used, from 1 to 3% by weight are preferred. Examples of suitable compounds are acrolein, methacrolein, diacetoneacrylamide, diacetonemethacrylamide, 2-butanonemethacrylates and vinyl acetoacetate.

From 0.5 to 1 mole of a dihydrazide of an aliphatic dicarboxylic acid, preferably of a C₄ -C₁₆ -dicarboxylic acid, can be added to the dispersion per mole of carbonyl-containing monomer, as an additional crosslinking agent where compounds (e) are present. Examples of suitable dihydrazides are adipodihydrazide, glutarodihydrazide and succinodihydrazide.

The novel polymer dispersions can be prepared by conventional emulsion polymerization processes using initiators, such as hydrogen peroxide, sodium persulfate, potassium persulfate, organic peroxides or conventional redox initiator systems, regulators, such as mercaptans, emulsifiers/protective colloids, such as Na salts of C₁₂ -C₁₈ -paraffinsulfonic acids or salts of acidic sulfuric half esters of ethylene oxide (propylene oxide) adducts, and other conventional assistants, so that further information is not necessary.

The novel dispersions usually have solids contents of from 20 to 60, preferably from 40 to 50, % by weight and LT (light transmission) values of from 70 to 98%, preferably from 90 to 96%.

Wood finishes prepared from the novel polymer dispersions may contain conventional additives, such as waxes based on paraffin and polyethylene. The waxes are usually added as 20-60% strength by weight dispersions during or after the emulsion polymerization. The waxes used have softening points of about 50°-250° C., preferably 70°-130° C. The size of the wax particles may be from 0.01 to 0.5 μm.

To improve film formation, film forming assistants, such as butylglycol, mixtures of diisobutyl esters of adipic, glutaric and/or succinic acid and other solvents may be added in amounts of from 3 to 8% by weight.

Flatting agents, such as silica compounds, in amounts of from 0.5 to 2% by weight, and thickeners based on polyurethane and/or polyacrylates may also be present.

The wood finishes prepared using the novel dispersions can be processed by brushing, spraying and curtain coating.

Drying is carried out at room temperature but may also be effected at elevated temperatures of up to 80° C. in conventional tunnel dryers.

The coatings thus obtained are readily sandable and have good stacking properties and high resistance to chemical influences.

EXAMPLES Preparation of the polymer dispersions

The initially taken mixtures stated in each of Examples 1 to 4 were heated to 85° C., while stirring, in a reaction vessel equipped with four feed vessels. 5% by weight of each of the feeds 1 and 2 were added, and initial polymerization was carried out for 15 minutes. The remaining amount of feed 1 was then added in the course of 2.5 hours, and the remaining amount of feed 2 introduced in the course of 2.75 hours. When feed 1 was complete, polymerization was continued for a further 1.75 hours, after which the mixture was cooled to 20° C. The pH was brought to 8 with 25% strength by weight aqueous ammonia solution. In Examples 1 and 2, the particular feed 3 stated there was then added dropwise in the course of 5 minutes, and stirring was continued for 55 minutes. The feeds 4 described in Examples 1 to 4 were finally each introduced in the course of 15 minutes, while stirring. The characteristic properties of the polymer dispersions are shown in the relevant Examples.

EXAMPLE 1

    ______________________________________                                         Initially taken                                                                            550 g of water                                                     mixture     33.7 g of 28% strength by weight Na                                            salt of C.sub.12 -C.sub.14 -fatty alcohol ether                                sulfate                                                                        (ethylene oxide content: 2.5% by weight)                           Feed 1      510 g of water                                                                 33.7 g of 28% strength by weight Na                                            salt of C.sub.12 -C.sub.14 -fatty alcohol ether                                sulfate                                                                        (ethylene oxide content: 2.5% by weight)                                       14.2 g of acrylic acid                                                         14.2 g of methacrylic acid                                                     14.2 g of 1,4-butandiol diacrylate                                             18.9 g of 50% strength by weight                                               acrylamide in water                                                            18.9 g of diacetoneacrylamide                                                  802.4 g of n-butyl methacrylate                                                141.6 g of methyl methacrylate                                     Feed 2      170 g of water                                                                 4.7 g of sodium peroxydisulfate                                    Feed 3      13.9 g of adipodihydrazide                                         Feed 4      90 g of 35% strength by weight                                                 polyethylene wax emulsion                                                      (dropping point of the solid 130° C.)                       Characteristic properties                                                      Solids content (% by weight)                                                                        43.8                                                      Viscosity, DIN 4 cup at 23° C. (s)                                                           32                                                        LT value (%)         96                                                        (0.01% strength by weight solution of the dispersion)                          ______________________________________                                    

EXAMPLE 2

    ______________________________________                                         Initially taken                                                                            550 g of water                                                     mixture     29 g of 15% strength by weight sodium                                          laurylsulfate                                                                  24.2 g of 20% strength by weight                                               oxyethylated C.sub.8 -alkylphenol                                              (ethylene oxide content: 2.5% by weight)                           Feed 1      370 g of water                                                                 67.7 g of 15% strength by weight sodium                                        laurylsulfate                                                                  14.6 g of acrylic acid                                                         14.2 g of methacrylic acid                                                     10.9 g of divinylbenzene                                                       19.4 g of 50% strength by weight                                               acrylamide in water                                                            19.4 g of diacetoneacrylamide                                                  967 g of n-butylmethacrylate                                       Feed 2      170 g of water                                                                 4.3 g of sodium persulfate                                         Feed 3      13.9 g of adipodihydrazide                                         Feed 4      90 g of 35% strength by weight                                                 polyethylene wax emulsion                                                      (dropping point of the solid 130° C.)                       Characteristic properties                                                      Solids content (% by weight)                                                                        46.6                                                      Viscosity, DIN 4 cup at 23° C. (s)                                                           38                                                        LT value (%)         94                                                        (0.01% strength by weight solution of the dispersion)                          ______________________________________                                    

EXAMPLE 3

    ______________________________________                                         Initially taken                                                                            550 g of water                                                     mixture     33.7 g of 28% strength by weight Na                                            salt of C.sub.12 -C.sub.14 -fatty alcohol ether                                sulfate                                                                        (ethylene oxide content: 2.5% by weight)                           Feed 1      340 g of water                                                                 33.7 g of 28% strength by weight Na                                            salt of C.sub.12 -C.sub.14 -fatty alcohol ether                                sulfate                                                                        (ethylene oxide content: 2.5% by weight)                                       7.1 g of acrylic acid                                                          14.2 g of methacrylic acid                                                     18.9 g of 1,4-butanediol diacrylate                                            243 g of methylmethacrylate                                                    708 g of n-butylmethacrylate                                       Feed 2      160 g of water                                                                 4.7 g of sodium persulfate                                         Feed 3      --                                                                 Feed 4      90 g of 35% strength by weight                                                 polyethylene wax emulsion                                                      (dropping point of the solid 130° C.)                       Characteristic properties                                                      Solids content (% by weight)                                                                        46.7                                                      Viscosity, DIN 4 cup at 23° C. (s)                                                           28                                                        LT value (%)         95                                                        (0.01% strength by weight solution of the dispersion)                          ______________________________________                                    

EXAMPLE 4

    ______________________________________                                         Initially taken                                                                            550 g of water                                                     mixture     33.7 g of 28% strength by weight Na                                            salt of C.sub.12 -C.sub.14 -fatty alcohol ether                                sulfate                                                                        (ethylene oxide content: 2.5% by weight)                           Feed 1      340 g of water                                                                 33.7 g of 28% strength by weight Na                                            salt of C.sub.12 -C.sub.14 -fatty alcohol ether                                sulfate                                                                        (ethylene oxide content: 2.5% by weight)                                       14.2 g of acrylic acid                                                         14.2 g of methacrylic acid                                                     18.9 g of 50% strength by weight                                               acrylamide in water                                                            188.8 g of methyl methacrylate                                                 755.2 g of n-butyl methacrylate                                    Feed 2      160 g of water                                                                 4.7 g of sodium persulfate                                         Feed 3      --                                                                 Feed 4      90 g of 35% strength by weight                                                 polyethylene wax emulsion                                                      (dropping point of the solid 130° C.)                       Characteristic properties                                                      Solids content (% by weight)                                                                        45.0                                                      Viscosity, DIN 4 cup at 23° C. (s)                                                           25                                                        LT value (%)         91                                                        (0.01% strength by weight solution of the dispersion)                          ______________________________________                                    

EXAMPLE 5 Preparation of a wood finish

20 g of xylene,

10 g of 3,3,5-trimethylpentane-1,3-diol isobutyrate,

3 g of the Na salt of a sulfated oleic acid dibutylamide (about 40% strength by weight in water),

4 g of an antifoam based on mineral oil,

21 g of water,

25 g of a 55% strength by weight polyurethane thickener in butyldiglycol/water (reaction product of an oxyethylated tallow fatty alcohol with hexamethylene diisocyanate),

15 g of a flatting agent based on silica,

34 g of ethylene glycol mono-n-butyl ether,

40 g of diethylene glycol monoethyl ether,

806 g of polymer dispersion according to Example 1, 2, 3 or 4,

2 g of a C₁₆ -C₁₉ -oxoalcohol containing 35 propylene oxide and 5 ethylene oxide units,

20 g of 50% strength by weight Ca stearate dispersion in water and

20 g of a 35% strength by weight polyethylene wax emulsion (dropping point of the solid 130° C.)

were mixed at room temperature, with stirring, to give a wood finish.

Table 1 summarizes the test results for the wood surfaces (light oak) coated with this wood finish. The wood finish was applied by spraying on two coats (about 80 g/m² of finish in each case) and sanding between the coats, and was dried at room temperature.

                  TABLE 1                                                          ______________________________________                                         Test results for the coated wood surface                                                 1      2        3      4 (Comparison)                                ______________________________________                                         1.  Sandability good     very   good good                                                               good                                                  2.  Scratch     very     very   very good                                          resistance  good     good   good                                           3.  Cold check.sup.2)                                                                          100      100    100  100                                           +50° C. and                                                             -20° C. alter-                                                          nately for                                                                     1 hour each                                                                4.  Steam resis-                                                                               very     very   good poor                                          tance.sup.3)                                                                               good     good                                                  5.  Resistance  good to  good to                                                                               good adequate                                      to chemical very     very                                                      influences  good     good                                                      according to                                                                   DIN 68,860                                                                 ______________________________________                                          (Specific results are shown in Table 2)                                        .sup.2) Wood specimens (20 × 5.5 × 0.8 cm) were coated once        with the wood finish (μm wet, application by means of a frame) and wer      dried at 70° C. for 20 minutes, sanded (grade 240) and coated agai      as described above. The coated wood was stored for 1 week at 23° C      and 50% relative humidity and then stored for 1 hour at +50° C. in      a drying oven and for 1 hour at -20° C. in a chest freezer. A cycl      comprises in each case storage at +50° C. and -20°C. After       each cycle, the coating was examined for damage (cracking). The cold chec      indicates the number of cycles and is a measure of the flexibility of the      coating.                                                                       .sup.3) The coating was exposed to steam at 100° C. for 5 minutes.      Softening and blushing were checked (immediately and after redrying for 2      hours).                                                                  

                  TABLE 2                                                          ______________________________________                                         Resistance of chemical influences (specific results)                           Test agent according to                                                                      Duration Example.sup.a)                                          DIN 68,860    of action                                                                               1     2   3   4 (Comparison)                            ______________________________________                                          1 acetic acid                                                                               60 min   0     0   0   0                                          2 citric acid                                                                               60 min   0     0   0   0                                          3 sodium carbonate                                                                           2 min   0     0   0   2                                          4 ammonia water                                                                              2 min   0     0   0   2                                          5 ethyl alcohol                                                                             60 min   0     0   2   4                                          6 white wine, red wine                                                                       5 h     0     0   1   2                                         fortified wine                                                                  7 beer        5 h     0     0   0   0                                          8 cola drinks                                                                               16 h     0     0   0   0                                          9 instant coffee                                                                            16 h     2     2   2   3                                         10 black tea  16 h     0     0   1   2                                         11 blackcurrent juice                                                                        16 h     1     1   1   3                                         12 condensed milk                                                                            16 h     0     0   2   4                                         12 water      16 h     0     0   0   1                                         14 gasoline    2 min   0     0   0   0                                         15 acetone    10 s     0     0   3   4                                         16 ethylbutyl acetate                                                                        10 s     0     0   2   2                                         17 butter     16 h     1     1   0   1                                         18 olive oil  16 h     1     0   0   1                                         19 mustard     5 h     1     1   1   1                                         20 salt        5 h     0     0   0   0                                         21 onions      5 h     0     0   0   0                                         22 lipstick   16 h     4     4   3   4                                         23 disinfectant                                                                              10 min   0     0   1   2                                         24 cleaner.sup.b)                                                                            60 min   1     1   2   4                                         25 cleaning solution.sup.c)                                                                  60 min   0     0   0   0                                         ______________________________________                                          .sup.a) Rating according to DIN 53,230                                          0: no change                                                                   5: very great change                                                          .sup.b) Mixture of 70% by weight of Na dodecylbenzenesulfonate and 30% by      weight of alkylaryl polyglycol ether                                           .sup.c) 33% strength by weight solution of b) in water                    

We claim:
 1. An aqueous copolymer dispersion in which the copolymer consists of:a) from 85 to 98.5% by weight of methacrylates of C₁ -C₁₂ -monalcohols, b) from 0.5 to 4% by weight of acrylic acid, methacrylic acid or mixtures thereof, c) from 0.5 to 4% by weight of acrylamide, methacrylamide or mixtures thereof, from 0.5 to 4% by weight of a diacrylate or a dimethacrylate of C₂ -C₂₄ -diols, a divinyl aromatic compound, a divinyl urea, a diallyl or triallyl compound or mixtures thereof, and e) from 0 to 3% by weight of acrolein, methacrolein, diacetoneacrylamide, diacetonemethacrylamide, 2-butanonemethacrylates, vinyl acetoacetate or mixtures thereof.
 2. The aqueous copolymer dispersion of claim 1, containing conventional additives.
 3. The aqueous copolymer dispersion of claim 2 wherein said additives are selected from waxes, film formation assistants, solvents, flattening agents, thickeners or mixtures thereof.
 4. An aqueous polymer dispersion as defined in claim 1, wherein the copolymer consists ofa) from 85 to 98.5% by weight of a mixture of N-butyl methacrylate and methyl methacrylate, b) from 0.5 to 4% by weight of a mixture of acrylic acid and methacrylic acid, c) from 0.5 to 4% by weight of acrylamide, and d) from 0.5 to 4% by weight of 1,4-butanediol diacrylate. 